A metal detection apparatus is used to detect and reject unwanted metal contamination. When properly installed and operated, it will help reducing metal contamination and improving food safety. Most modern metal detectors utilise a search head comprising a “balanced coil system”. Detectors of this design are capable of detecting all metal contaminant types including ferrous, nonferrous and stainless steels in a large variety of products such as fresh and frozen products.
The metal detection apparatus typically comprises a metallic enclosure, having entrance and exit apertures with cross-sectional areas of different or equal size defining a travel path inside the enclosure along which an object under inspection moves.
A metal detection apparatus that operates according to the “balanced coil”-principle typically comprises three coils that are wound onto a non-metallic frame or yoke. The coil system comprises at least one transmitter coil and at least one first and at least one second receiver coil; the respective receiver coils bounding a detection zone inside the enclosure between the entrance and exit apertures. The detection zone has a cross-sectional profile that varies or is constant along the travel path. Systems with cylindrical detection zones typically use coils having identical dimensions with the transmitter coil centred between the two receiver coils. Systems with conical detection zones use coils that differ in size from one another typically with the transmitter coil being off-centred between the two receiver coils. In both systems the coils are arranged such that, when the at least one transmitter coil is energized by an alternating electric current, the electromagnetic field generated thereby induces a first voltage in the first receiver coil and a second voltage in the second receiver coil, the first and second voltages cancelling each other out when there is no metal present in the object under inspection.
As a particle of metal passes through the coil arrangement, the high frequency field is disturbed first near one receiver coil and then near the other receiver coil. While the particle of metal is conveyed through the receiver coils the voltage induced in each receiver coil is changed. This change in balance results in a signal at the output of the receiver coils that can be amplified, processed and subsequently be used to detect the presence of the metal contamination.
The signal processing channels typically split the received signal into two separate components that are 90° apart from one another. The resultant vector has a magnitude and a phase angle, which is typical for the products and the contaminants that are conveyed through the coil system. In order to identify a metal contaminant, “product effects” need to be removed or reduced. If the phase of the product is known then the corresponding signal vector can be reduced. Eliminating unwanted signals from the signal spectrum thus leads to higher sensitivity for signals originating from contaminants.
Methods applied for eliminating unwanted signals from the signal spectrum therefore exploit the fact that the product, the contaminants and other disturbances, have different influences on the magnetic field so that the resulting signals differ in phase.
Distinguishing between the phases of the signal components of different origin by means of a phase detector allows obtaining information about the product and the contaminants. A phase detector, e.g. a frequency mixer or analogue multiplier circuit, generates a voltage signal which represents the difference in phase between the signal input, such as the signal from the receiver coils, and a reference signal provided by the transmitter unit to the receiver unit. Hence, by selecting the phase of the reference signal to coincide with the phase of the product signal component, a phase difference and a corresponding product signal is obtained at the output of the phase detector that is zero. In the event that the phase of the signal components that originate from the contaminants differ from the phase of the product signal component, then the signal components of the contaminants can be detected. However in the event that the phase of the signal components of the contaminants is close to the phase of the product signal component, then the detection of contaminants fails, since the signal components of the contaminants are suppressed together with the product signal component.
In known systems the transmitter frequency is therefore selectable in such a way that the phase of the signal components of the metal contaminants will be out of phase with the product signal component.
WO 2012/045578A1 discloses a method for operating a metal detection system that allows determining a preferable transmitter frequency with which signal components of smallest sized metal particles differ most in phase and amplitude from the phase and amplitude of a product signal.
Hence, for testing and optimising a metal detection apparatus tests with different contaminants need to be performed, which is however time-consuming. Furthermore, tuning should be performed regularly particularly of the changes of the processed product.
U.S. Pat. No. 5,160,885 A discloses a testing device for testing a metal detecting apparatus wherein a test article of representative metal is passed within the coil system through a detection zone for recording a test signal that corresponds to response signals caused by metal particles included in a product travelling through the detection zone. The test article is guided internally in a non-metallic guide tube, which extends through the detection zone from an input aperture to an output aperture of the metal detection apparatus.
The test article is embedded in a slug, which can move freely within the guide tube from one end to the other, and is constrained to move therealong, by pulses of air introduced through the air pipes. Upon the application of pulses of air first through one junction and then through the other junction, the slug will travel from left to right and then from right to left. The test articles can be moved with or without a product simultaneously passing through the detection zone. Hence, at both sides of the metal detection apparatus, end junctions of the tubular guide are connected to pneumatic fittings and pipes.
The guide tube and the pneumatic equipment provided at both ends require considerable space, which is sparsely available in typical metal detection devices. Due to limitations in space, installation of this test device has been avoided for example in metal detection devices, which use conical detection zones.
In food environments, pneumatic elements and fittings of the test device act as dirt traps and require cleaning and maintenance. Further, these items are obstacles and may be subject to accidental damage, e.g. when products are handled.
Further, assembling a metal detection apparatus together with such a test device requires, besides the pneumatic equipment, considerable efforts. Installing the test device into a metal detection apparatus that is already operating in the field is scarcely possible since space is not available for the rather voluminous test device.
U.S. Pat. No. 5,994,897 A discloses a metal detection apparatus which uses the test device disclosed in U.S. Pat. No. 5,160,885 A. A frequency generator, which generates the signal that is supplied to the transmitter coil, is capable of operating on numerous frequencies in the 50 kHz to 2 MHz range. The product is passed through the detector head, with and without the test article present, for each of the operating frequencies. The response signal from the product and the response signal from the product with metal are evaluated for each of the operating frequencies. In this manner the frequency which produces the highest ratio of the signal of the contaminated product compared to the signal of the uncontaminated product can be identified. However, again, this process of optimising the metal detecting apparatus is extremely laborious.
The present invention is therefore based on the object of providing an improved metal detection apparatus that is equipped with an improved test device, the improved test device and an improved method for optimising the metal detection apparatus.